Microporous films or membranes have a structure that enables fluids to flow through them. The effective pore size is at least several times the mean free path of the flowing molecules, namely from several micrometers down to about 100 Angstroms. Such sheets are generally opaque, even when made of a transparent material, because the surfaces and the internal structure scatter visible light.
Microporous membranes or films have been utilized in a wide variety of applications such as for the filtration of solids, the ultra filtration of colloidal matter, as diffusion barriers or separators in electrochemical cells, or in the preparation of synthetic leather, and in cloth laminates. The latter utilities require, of course, permeability to water vapor but not liquid water for the applications as synthetic shoes, raincoats, outer wear, camping equipment such as tents, and the like. Microporous films or membranes are often utilized for filter cleaning antibiotics, beer, oils, bacteriological broths, as well as for the analysis of air, microbiological samples, intravenous fluids, vaccines and the like. Microporous membranes or films are also utilized to make surgical dressings, bandages, and in other fluid transmissive medical applications. The microporous membrane or film may be laminated to other articles to make laminates having particular utility. Such laminations may include a microporous layer and an outer shell layer to provide a particularly useful garment material. The microporous films or membranes may be utilized as a tape backing to provide products such as a vapor-transmissive wound dressing or hair set tape.
The art of preparing microporous films or membranes is not restricted, but rather is replete with a wide variety of methods of producing such articles. Methods commonly used to provide microporous films or membranes are described in the following references:
U.K. Patent Application GB No. 2,026,381 A, published February 6, 1980, discloses the preparation of membranes having a porous surface by blending polymer with a liquid component to form a binary two-phase system which, in the liquid aggregate state, has a region of miscibility and a region with miscibility gaps, forming a sheet of the blend in the former state, casting the film into a bath of the liquid component, and removing the liquid component to provide porosity. The resultant non-oriented porous sheet has a relatively low tensile strength.
Various patents assigned to W. L. Gore and Associates, Inc., including U.S. Pat. Nos. 3,953,566; 3,962,153; 4,096,227; 4,110,392; 4,187,390 and 4,194,041, describe the preparation of porous articles, including microporous sheets formed exclusively of polytetrafluoroethylene (PTFE), not a conventional thermoplastic polymer, characterized by having polymer nodes connected by fibrils. Such articles are produced by extruding a paste comprised of PTFE particles and a lubricant, removing the lubricant, and stretching and annealing the resultant product. The resultant product is a sintered, oriented porous film of PTFE.
U.S. Pat. No. 3,201,364, assigned to Monsanto Company, describes the preparation of a non-oriented, non-porous packaging film comprised of a single phase blend of polypropylene and mineral oil.
U.S. Pat. Nos. 4,100,238 and 4,197,148 describe the preparation of microporous films by extruding a two component blend, solvent leaching one dispersed component and stretching the resultant leached film to obtain a desired porosity. The blend consists of polymer and a leachable, non-miscible substance. Once the leachable dispersed phase is removed and the film oriented, a porous film results.
U.S. Pat. No. 3,679,540, assigned to Celanese Corporation, discloses a method for making a microporous polymer film by cold stretching an elastic polymer film until porous surface regions are formed by film failure, hot stretching the cold stretched film until fibrils and pores or open cells are formed and then heat setting the resultant film. Controlled porosity is generally not attained in such films because they do not always uniformly fibrillate to a specific pore size.
Certain U.S. patents disclose the preparation of porous films by blending into the polymer a non-miscible leachable particulate substance such as starch, salts, etc. forming a sheet and leaching the particulate substance from the polymer sheet. Such U.S. patents include U.S. Pat. Nos. 3,214,501 and 3,640,829. U.S. Pat. No. 3,870,593 discloses the preparation of a microporous polymer sheet by blending non-miscible, non-leachable filler into the polymer, forming a sheet of the blend and stretching the sheet to form pores which are initiated at the sites of the filler particles.
U.S. Pat. No. 4,247,498 (Castro) discloses a process for making microporous polymers from blended mixtures of polymer and compatible liquid which are cooled under nonequilibrium thermodynamic conditions to initiate liquid-liquid phase separation. This involves cooling the heated homogeneous solution of polymer and compatible liquid so that a droplet of liquid will form surrounded by liquid polymer. Further cooling produces a solid phase which contains liquid which is removed, forming a structure characterized by a series of enclosed cells having substantially spherical shapes and pores interconnecting adjacent cells.